Open Stack

On Tue, May 19, 2020 at 23:48, Sean Mooney <smooney at redhat.com> wrote:
> On Tue, 2020-05-19 at 21:55 +0200, Slawek Kaplonski wrote:
>>  Hi,
>> 
>>  Thx for starting this thread.
>>  I can share some thoughts from the Neutron point of view.
>> 
>>  On Tue, May 19, 2020 at 04:08:18PM +0200, Balázs 
>> Gibizer wrote:
>>  > Hi,
>>  >
>>  > [This is a topic from the PTG etherpad [0]. As the PTG time is 
>> intentionally
>>  > kept short, let's try to discuss it or even conclude it before 
>> the PTG]
>>  >
>>  > As a next step in the minimum bandwidth QoS support I would like 
>> to solve
>>  > the use case where a running instance has some ports with minimum 
>> bandwidth
>>  > but then user wants to change (e.g. increase) the minimum 
>> bandwidth used by
>>  > the instance.
>>  >
>>  > I see two generic ways to solve the use case:
>>  >
>>  > Option A - interface attach
>>  > ---------------------------
>>  >
>>  > Attach a new port with minimum bandwidth to the instance to 
>> increase the
>>  > instance's overall bandwidth guarantee.
>>  >
>>  > This only impacts Nova's interface attach code path:
>>  > 1) The interface attach code path needs to read the port's 
>> resource request
>>  > 2) Call Placement GET /allocation_candidates?in_tree=<compute RP 
>> of the
>>  > instance>
>>  > 3a) If placement returns candidates then select one and modify 
>> the current
>>  > allocation of the instance accordingly and continue the existing 
>> interface
>>  > attach code path.
>>  > 3b) If placement returns no candidates then there is no free 
>> resource left
>>  > on the instance's current host to resize the allocation locally.
> so currently we dont support attaching port with resouce request.
> if we were to do that i would prefer to make it more generic e.g. 
> support attich sriov devices as well.
> 
> i dont think we should ever support this for the usecase of changing 
> qos policies or bandwith allocations
> but i think this is a good feature in its own right.
>>  >
>>  >
>>  > Option B - QoS rule update
>>  > --------------------------
>>  >
>>  > Allow changing the minimum bandwidth guarantee of a port that is 
>> already
>>  > bound to the instance.
>>  >
>>  > Today Neutron rejects such QoS rule update. If we want to support 
>> such
>>  > update then:
>>  > * either Neutron should call placement allocation_candidates API 
>> and the
>>  > update the instance's allocation. Similarly what Nova does in 
>> Option A.
>>  > * or Neutron should tell Nova that the resource request of the 
>> port has been
>>  > changed and then Nova needs to call Placement and update 
>> instance's
>>  > allocation.
>> 
>>  In this case, if You update QoS rule, don't forget that policy with 
>> this rule
>>  can be used by many ports already. So we will need to find all of 
>> them and
>>  call placement for each.
>>  What if that will be fine for some ports but not for all?
> i think if we went with a qos rule update we would not actully modify 
> the rule itself
> that would break to many thing and instead change change the qos rule 
> that is applied to the port.
> 
> e.g. if you have a 1GBps rule and and 10GBps then we could support 
> swaping between the rules
> but we should not support chnaging the 1GBps rule to a 2GBps rule.
> 
> neutron should ideally do the placement check and allocation update 
> as part of the qos rule update
> api action and raise an exception if it could not.
>> 
>>  >
>>  >
>>  > The Option A and Option B are not mutually exclusive but still I 
>> would like
>>  > to see what is the preference of the community. Which direction 
>> should we
>>  > move forward?
>> 
>>  There is also 3rd possible option, very similar to Option B which 
>> is change of
>>  the QoS policy for the port. It's basically almost the same as 
>> Option B, but
>>  that way You have always only one port to update (unless it's not 
>> policy
>>  associated with network). So because of that reason, maybe a bit 
>> easier to do.
> 
> yes that is what i was suggesting above and its one of the option we 
> discused when first
> desigining the minium bandwith policy. this i think is the optimal 
> solution and i dont think we should do
> option a or b although A could be done as a sperate feature just not 
> as a way we recommend to update qos policies.

My mistake. I don't want to allow changing a rule I want to allow 
changing which rule is assigned to a bound port. As Sean described this 
direction might require neutron to call GET /allocation_candidates and 
then update the instance allocation as a result in placement. However 
it would create a situation where the instance's allocation is managed 
both from nova and neutron.

> 
>> 
>>  >
>>  >
>>  > Both options have the limitation that if the instance's current 
>> host does
>>  > not have enough free resources for the requested change then Nova 
>> will not
>>  > do a full scheduling and move the instance to another host where 
>> resource is
>>  > available. This seems a hard problem to me.
> i honestly dont think it is we condiered this during the design of 
> the feature with the
> intent of one day supporting it. option c was how i always assumed it 
> would work.
> support attach and detach for port or other things with reqsouce 
> requests is a seperate topic
> as it applies to gpu hotplug, sriov port and cyborg so i would ignore 
> that for now and focuse
> on what is basicaly a qos resize action where we are swaping between 
> predefiend qos policies.
>>  >
>>  > Do you have any idea how can we remove / ease this limitation 
>> without
>>  > boiling the ocean?
>>  >
>>  > For example: Does it make sense to implement a bandwidth weigher 
>> in the
>>  > scheduler so instances can be spread by free bandwidth during 
>> creation?
> we discussed this in the passed breifly. i always belived that was a 
> good idea but it would require the allocation
> candiates to be passed to the weigher and the provider summaries. we 
> have other usecases that could benifit form that
> too but i think in the past that was see as to much work when we did 
> not even have the basic support working yet.
> now i think it would be a resonable next step and as i said we will 
> need the ability to weigh based on allcoation
> candiates in the future of for other feature too so this might be a 
> nice time to intoduce that.
>>  >
>>  >
>>  > Cheers,
>>  > gibi
>>  >
>>  >
>>  > [0] https://etherpad.opendev.org/p/nova-victoria-ptg
>>  >
>>  >
>>  >
>> 
>> 
>